1. Field of the Invention
The present invention relates generally to vacuum pumping systems, and more particularly to a two stage cyropump with an improved first stage.
2. Discussion of the Prior Art
Several semiconductor fabrication processes such as thin film sputtering and etching are carried out in work chamber environments of mixtures of inert (e.g. argon) carrier gas and one or more process gas(es) at partial pressures which must be precisely controlled. Referring to FIG. 1, a work chamber 10 environment of specified gases is typically established by first using a mechanical vacuum pump 12 connected through pipe 13 to evacuate chamber 10 as much as possible, and then using source 14 of the specified gases to purge the chamber. Work chamber gas pressures may then be adjusted by exposure to and capture on chilled surfaces in a condenser or cryopump 20 which in effect pumps the condensed gases from the work chamber. A conventional cryopump 20 includes a housing 22 with walls 24 having inner surfaces 25 forming a vacuum envelope or cavity 26 with a top flange 27 around an opening 28 for fluid communication through a lid or base plate 29 and through conduit 30 to work chamber 10. Housing 22 has a floor 31 with a port 32 seated around an elongated "cold finger" or cooling means 34 which extends up from an external "cold head" 36. Cold finger 34 receives helium gas at room temperature and high pressure from a compressor (not shown) in a closed cycle gaseous helium refrigeration system. Cold finger 34 has first expansion cylinder 41 and smaller diameter telescoping second expansion cylinder 42 projecting into cavity 26 for expanding received helium gas to lower pressures and lower temperatures to chill cryopump surfaces to condense gases from the work chamber.
Work chamber residual water vapor commonly slows and limits the evacuation of the chamber and hinders carrying out various processes in the chamber. Many process gases are used at relatively low but critical pressures in the chamber, and will only condense at extremely low temperatures. To prevent water vapor from condensing, on and interfering with condensation of other gases on, surfaces needed at these lower temperatures, a conventional cryopump 20 uses a first stage 44 to condense water vapor and a second stage 46 to condense other gases. First stage 44 includes a shroud 48 supported solely by, and in thermal contact with, first stage expansion cylinder 41 of cold finger 34. Shroud 48 inside surfaces 50 form a compartment 51 surrounding the second stage expansion cylinder 42 and having a throat 52 which opens upwardly towards cavity opening 28. Compartment 51 contains panel, array or baffle means 54, which is thermally connected through couplings 56 and shroud 48 to first stage cylinder 41. Baffle 54 is typically formed by chevron shaped louvers 58 in an optically dense arrangement which blocks all possible lines of sight through baffle 54 and obstructs the compartment throat 52, obliging entering gas such as water vapor to wend around, and probably condense on, louver 58 surfaces chilled to between 77 and 90 degrees Kelvin, before being able to pass through and exit downwards into compartment 51 and second stage 46. First stage baffle 54 buffers or shield substantial thermal radiation from penetrating to second stage 46, but must allow uncondensed gases to have access to the inner second stage 46 of the cryopump.
Second stage 46 includes panel 60 supported on, and in thermal contact with, the distal end of second stage expansion cylinder 42, which chills panel 60 to about 18 degrees Kelvin to condense most remaining gases. Second stage 46 typically further includes an adsorbent bed 62 of charcoal granules or bonded sieve material to remove noncondensible gases such as hydrogen, helium, and neon.
The work chamber gas pressure can be controlled by a second stage 46 at a given temperature by controlling the second stage pressure by "throttling" the entering gas flow from work chamber 10. This has been done by "cranking" main valve 64 in conduit 30 upstream of cryopump 20, but throttling an upstream valve 64 also restricts the entry of water vapor, and hence reduces the water vapor pressure and condensation rate in the first stage 44 of cryopump 20.
U.S. Pat. No. 4,094,492 to Beeman describes a variable iris aperture used to control a flow of gas in a connecting conduit and thereby control the pressure in a differential vacuum (diffusion) pumping system (not shown). Before reaching the iris valve, water vapor is condensed in the conduit upstream by a liquid nitrogen cold trap. Vacuum Magazine, Vol. 34 No. 7 (1984), discloses a similar arrangement for a diffusion pumping system.
FIG. 2 illustrates schematically how in both U.S. Pat. No. 4,285,710 to Welch and U.S. Pat. No. 4,531,372 to Slabaugh the flow of gas into and pressure in a cryopump 70 second stage 46 is throttled by using a valve 72. The valve 72 body and vanes or rotor (not shown in detail) form an integral part of first stage 74, and, in all opening positions, expose a substantially constant surface area which is chilled to condense water vapor as a substitute for a conventional first stage condenser baffle 54. However, the valve 72 surface areas exposed are not actually constant and do not condense water at full speed independently of the valve positions. Such "constant" area valves do not dynamically and precisely control pressure over a wide range. These substitute first stage condenser valves 72 have mechanical support linkages 75 to pump housing 76, through which heat from the ambient surroundings is absorbed. Valves 72, through mounting blocks 77, are thermally connected to the first stage shroud 78. Valves 72 thereby constitute extra thermal loads and require extra refrigeration capacity for first stage 74 to condense water vapor. Welch provides a cryopump 70 with a liquid nitrogen reservoir (shown in dashed outline) to augment chilling first stage valve 72 and shroud 78. However, liquid nitrogen is undesireably expensive and inconvenient for condensing residual water vapor upstream of the second stage.
Thus, there is a need for a means for controlling the rate of gas flow into the second stage of cryopump while maintaining the efficiency and speed of condensing water vapor in the first stage of the cryopump.